In general, when etching of titanium is carried out, a hydrofluoric acid-based etching solution is used. However, when such a hydrofluoric acid-based etching solution is used, the underlying base material will also be etched in cases where the underlying base material is nickel or glass.
Furthermore, when etching of titanium is carried out, a sodium hydroxide solution may be used. However, titanium will hardly be etched with such an etching solution alone, and the etching rate is very slow even when hydrogen peroxide is combined with the etching solution to improve the etching rate. Therefore, this etching solution is impracticable.
In recent years, as for etching solutions for titanium, a method in which EDTA is used as a complexing agent has been known, and, specifically, a method in which an alkaline etching solution containing EDTA and hydrogen peroxide is used has been known (Patent Document 1).
However, while titanium deposited on lithium niobate (LiNbO3) can be selectively etched by this method, there is a problem of slow etching rate. Furthermore, in this method, some substances (metals) cannot be etched, and the underlying base material may be adversely affected depending on the type of the underlying base material. For example, since the etching rate of titanium is slower than the etching rate of nickel, the underlying base material is dissolved prior to dissolution of titanium.